Caption: Water milfoil stem. Light micrograph of a transverse section through the stem of the aquatic whorled water milfoil (Myriophyllum verticillatum) plant. All aquatic plants (hydrophytes) have a similar stem structure. The epidermis has a thin cuticle and is permeable to water. The vascular tissue (centre) acts as a cable to resist the movement of water. Below the outer endodermis (purple ring) is a ring of phloem (small dark blue cells) and an inner thin ring of xylem (large circles). The pith consists of large-celled parenchyma (deep blue). Magnification: x14 when printed 10 centimetres wide.